The Pacific in a normal year: the Walker circulation

Overview

El Niño Southern Oscillation, upwelling off Peru in a normal year and walker circulation.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is the El Niño Southern Oscillation?

   Model answer

   It is a coupled ocean and atmosphere cycle in the tropical Pacific in which sea surface temperatures and surface air pressure swing between phases every few years. El Niño refers to the ocean part, the appearance of unusually warm water in the central and eastern Pacific; the Southern Oscillation refers to the atmospheric part, a see-saw of air pressure between the western and eastern Pacific. The two normally move together, which is why they are treated as one system, and its effects reach far beyond the Pacific.

2. 2

   Describe what the trade winds do across the Pacific in a normal year.

   Model answer

   Strong and steady south-east and north-east trade winds blow from the subtropical high pressure areas towards the equator and then westwards along it, from South America towards Indonesia and Australia. As they blow they drag the warm surface water of the ocean with them, piling it up in the western Pacific. This westward push is the engine of the whole normal pattern, and everything else follows from it.

3. 3

   Describe conditions in the western Pacific, around Indonesia and northern Australia, in a normal year.

   Model answer

   Warm surface water driven west by the trade winds accumulates here as a deep warm pool, with sea surface temperatures of about 28 to 30 degrees Celsius and sea level standing perhaps half a metre higher than in the east. The very warm sea heats the air above it, which becomes buoyant and rises, so surface pressure is low. The rising air is laden with moisture, so it condenses into towering cumulonimbus and delivers heavy convectional rainfall to Indonesia, Papua New Guinea and northern Australia.

4. 4

   Describe conditions off the coast of Peru and Ecuador in a normal year.

   Model answer

   The trade winds have dragged the surface water away to the west, so the warm layer here is thin and cold water rises from depth to replace it. Sea surface temperatures are only about 20 to 22 degrees Celsius, much cooler than the western Pacific. The cool sea chills the air above it, making it dense and stable, so surface pressure is high, air subsides, and there is almost no rainfall, which is why the coastal strip of Peru and northern Chile is desert. Fishing, however, is excellent.

5. 5

   What is upwelling off Peru in a normal year, and why does it matter economically?

   Model answer

   As surface water is dragged offshore, cold water from 100 metres or more below rises to take its place. This deep water is rich in nitrates and phosphates that have sunk from the surface over time, so when it reaches the sunlit layer it fertilises enormous blooms of phytoplankton. These support one of the largest fisheries on Earth, the Peruvian anchoveta fishery, together with vast colonies of seabirds whose droppings are harvested as guano. Fishing and fishmeal are a major part of the Peruvian economy.

6. 6

   What is the thermocline, and how does it lie across the Pacific in a normal year?

   Model answer

   The thermocline is the boundary layer in the ocean across which temperature drops sharply from the warm surface water above to the cold deep water below. In a normal year it is tilted: it lies deep, at about 150 to 200 metres, beneath the piled-up warm pool of the western Pacific, and shallow, at about 40 to 50 metres, off South America. Because it is shallow in the east, the winds are able to draw cold water from below it up to the surface.

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